Process control is a long-established art which plays a major role in managing industrial plants and processes. In this art, process transmitters have been used to monitor process variables. Having evolved from the earliest measurement devices such as barometers and thermometers, the process transmitter has traditionally received a great deal of technological attention to improve performance due to the need for accurate process measurement. Since the accuracy of every measurement made in a process control loop is directly dependent upon the accuracy of the particular process transmitter or instrument which closes the loop, the process transmitter plays a particularly sensitive role in industrial process control systems.
Beginning in the 1950s, electrical and electronic process control loops were a natural development from prior electromechanical control systems. The general problem of electronic process control is to convert a physical variable to an electrical signal, and to subsequently transmit that signal to a recorder and/or other control equipment which may be located some distance away from the physical variable. Early types of process control loops to accomplish this goal were "four-wire" systems, and were configured such that operating power was supplied through two of the four wires and a process signal was transmitted through the other two wires. The four-wire system requires the use of amplifiers or other signal conditioning equipment at the point of measurement in order to supply an accurate signal representative of the physical variable since the process signal is generally very low. See, e.g., U.S. Pat. No. 3,680,384, of Grindheim. Prior four-wire transmitter systems thus required separate power supply lines, and voltage power supplies.
After the four-wire transmitter was developed, it became apparent that the advantages of using the same two wires for power supply and information transmission would greatly improve the process control art. The "two-wire" transmitter was then developed and operates today in a control loop in conjunction with an external power supply, a pair of wires from the supply, and a transmitter connected serially between the wires. As used herein, the term "two-wire" is construed broadly to mean two conductors. Thus, the term "two-wire" includes actual wires, twisted pairs, coaxial cables, and other pairs of conductors.
During operation of such a two-wire transmitter loop, the transmitter energizes a sensor element and receives informational signals from the sensor element. The information is transmitted on the pair of wires by varying the current in the current loop. Thus the transmitter acts as a variable current sink, and the amount of current which it sinks is representative of the information from the sensor. Such prior two-wire transmitter loops have generally been analog in nature, and the industry standard which has developed for two-wire transmitters is a 4 to 20 milliamp loop, with a variable loop supply voltage having a maximum output of 42 volts DC. With such a low voltage supply, two-wire transmitter loops are particularly suited for use in hazardous environments. See, e.g., U.S. Pat. No. 4,242,665, of Mate.
More advanced prior two-wire transmitter control loops exhibit high-level data communication between two-wire transmitters and various receiving elements, for example controllers and communication devices. The concept of digital communication in 4 to 20 milliamp control systems is known for use in the more complicated 4 to 20 milliamp loops having both digital and analog components. Transmitters suitable for such purposes are usually called "smart" transmitters because they are more accurate and have operating parameters which may be remotely controlled.
The trend in two-wire transmitter loops both in the smart, microprocessor-based transmitter area and the traditional analog transmitter area, has been to reduce the power requirements for components which are used in the loop. This need has arisen since the amount of power which a two-wire transmitter may draw from a current loop to use for its operation is severely limited. With a nominal 10-volt supply, at the bottom end of operation only about 40 milliwatts is available to power any instrumentation in the loop. Thus with large power demands on the loop, two-wire control systems may be limited to a few low power industrial control applications. This aspect of industrial controls competes with the general desire to design instrumentation into the loop to simplify loop operation and installation, and to provide intrinsic safety in a low power process control environment.
This long-felt need has not adequately been met by process control loops which have the aforementioned inherent power budget problems. Since only 40 milliwatts of power are available to run the circuitry in the transmitter and the loop, power supply circuits have been developed which attempt to minimize power loss in the circuit and provide steady power levels to the control loop. Traditional methods of supplying low power to electronic circuitry include the well known "flyback regulator control" power supply circuits wherein the pulse width of the output current is based on a flyback voltage developed across an inductor in the circuit. In this type of power supply circuit, a pass transistor is usually turned on and the inductor current is allowed to rise until a threshold is reached turning off the pass transistor.
In flyback regulator control circuits, generally two methods have been used to sense the inductor current. The flyback inductor saturation current may be sensed, or a shunt resistor can be placed in series with the flyback inductor to directly sense the state of the flyback inductor current. However, both of these current sensing techniques introduce a large amount of power loss to the power supply, and therefore are unacceptable for use in low power 40 milliwatt systems. Prior power supplies using these methods of sensing the inductor current simply do not fulfill a long-felt need in the art for low power loss switching power supplies for electronic circuitry.
Two-wire transmitters are often remotely located. The length of the wires connecting components in the loop can exceed 5000 feet, and the longer wires have higher resistance. This resistance reduces the voltage to the transmitter. Other devices such as indicators, recorders and barriers are usually added to the loop, further increasing the loop resistance. Large amounts of loop resistance cause a wide variation in the input voltage. Changes of load current can vary widely also as different digital systems are accessed. The combination of high line variation and high load variation place a difficult burden on the traditional methods of switching power supply control circuits which have the further requirement of low startup current.
It is thus important in designing low power electronic systems, and particularly two-wire transmitters for use in process control loops, to provide switching power supplies having low loss regulated voltage control. These power supplies should provide a steady output to run the electronic circuitry in the system, and should provide reliable output voltages which are dependable for use in sensitive electronic instrumentation. Furthermore, these power supplies should have high efficiencies so that the power output is continuously available to the system. These goals have not previously been achieved in the switching power supply art.